1. Field of the Invention
The present invention relates to an exhaust manifold, and more particularly to an insulated exhaust manifold for an internal combustion engine in a motor vehicle.
2. Description of Related Art
Catalytic converters in motorized vehicles, particularly passenger automobiles, must reach a certain temperature before they xe2x80x9clight offxe2x80x9d. Light off occurs when the catalytic converter begins to convert harmful pollutants by oxidizing carbon monoxide and hydrocarbons to CO2, and reducing NOx to N2 and O2. It is important to minimize the time to light off once a car is started to minimize the amount of harmful pollutants emitted to the atmosphere.
Catalytic converters are typically heated to light off by the high temperature engine exhaust gas itself. Unfortunately, the catalytic converter is normally mounted downstream of the exhaust manifold which conducts the heated exhaust gas from the engine. A typical exhaust manifold is made of metal, or substantially made of metal. Metal exhaust manifolds conduct and disperse thermal energy away from exhaust gas to the outside atmosphere. This loss in thermal energy reduces the exhaust gas temperature before it reaches the catalytic converter and delays light off.
Various techniques for insulating exhaust manifolds and/or for providing other means to speed up light off have been suggested and attempted. Cast iron exhaust manifolds are useful but heavy. Also, the mass (large thermal mass) of iron drains heat from the exhaust gas. Welded tubing exhaust manifolds have less mass, but are complicated and expensive. Double-walled welded tubing exhaust manifolds have been suggested, with an air gap between the walls, but the two walls have the same thickness and are both structural and such an exhaust manifold would be unreasonably complex to manufacture.
U.S. Pat. No. 5,419,127 teaches an exhaust manifold having inner and outer metal walls enclosing a layer of insulating material. Because the inner layer is metal, and has finite thermal mass, it conducts heat from the traveling exhaust gas thus delaying light off. In addition, the metal inner layer is subject to erosion or loss of integrity over time from thermal cycling.
There is a need in the art for an exhaust manifold that substantially reduces the amount of heat conducted or convected away from the exhaust gas. Such an improved manifold will provide higher temperature exhaust gas to the catalytic converter, thus minimizing the time from engine start-up to light off.
An exhaust manifold is provided having a ceramic inner layer which defines an exhaust gas passageway of the manifold, a ceramic insulation layer disposed exterior to and adjacent the inner layer, and an outer structural layer disposed exterior to and adjacent the insulation layer. Each of the inner and insulation layers of the manifold comprises ceramic fibers and ceramic filler material.
A method of making an exhaust manifold is also provided having the steps of: a) providing a first aqueous slurry having 1-2 wt. % solids, wherein the solids comprise a mixture of ceramic filler material and ceramic fibers; b) vacuum forming an insulation layer from the first aqueous slurry; c) providing a second aqueous slurry having 1-2 wt. % solids, wherein the solids comprise a mixture of ceramic filler material and ceramic fibers; d) vacuum forming an inner layer on an interior wall surface of the insulation layer; e) firing the formed insulation and inner layers to provide finished insulation and inner layers respectively; and f) casting a metal outer layer over the insulation layer to form the exhaust manifold.
An exhaust manifold is also provided having a substantially ceramic integrated layer defining an exhaust gas passageway of the manifold, and an outer structural layer disposed exterior to and adjacent the integrated layer. The integrated layer comprises ceramic fibers and ceramic filler material, and has a radial porosity gradient such that localized porosity increases in an outward radial direction in the integrated layer.
A method of making an integrated layer having a radial porosity gradient for an exhaust manifold is also provided. The method has the steps of: a) providing a plurality of aqueous slurries, the slurries comprising a solids mixture of ceramic fibers and ceramic filler material and having incrementally increasing filler:fiber ratios; b) vacuum forming the integrated layer by successively introducing the slurries into a vacuum formation process to provide an integrated layer having a radial porosity gradient.
A method of making an exhaust manifold using a moldable dough is also provided. The method has the following steps: a) combining ceramic fibers, ceramic filler material, a binder, and water in the following proportions to form a dough paste:
b) mixing the dough paste to form a moldable dough; c) pressing the moldable dough into a mold to form a shaped part; d) curing or gelling the shaped part so that said shaped part can be handled without causing distortions; e) heating to burn off or remove residual organics and water, thereby forming a finished insulation layer; f) providing an aqueous slurry having 1-2 wt. % solids, said solids comprising a mixture of ceramic filler material and ceramic fibers; g) vacuum forming or spray-forming from the aqueous slurry an inner layer on an interior wall surface of the insulation layer; h) firing the formed insulation and inner layers to provide finished insulation and inner layers respectively; and i) casting a metal outer layer over the insulation layer to the exhaust manifold.